1. Field of the Invention
This invention relates to ultra-high vacuum systems and more particularly provides remotely actuatable and demountable sealing arrangements among cylindrical components of such systems.
2. Description of the Prior Art
One of the most effective sealing devices available is the rubber or elastomeric O-ring, which is widely used to seal components containing liquids and gases. In the most common application, the O-ring forms a seal between two concentric cylindrical surfaces. The ring is stretched over the inner cylinder and is compressed radially inward by the outer cylinder, so that its cross section, typically circular in the free state, is slightly flattened where it is in contact with the cylinders. For low and moderate pressure differentials these two circumferential contact zones are sufficient to form the seal. For higher pressures the O-ring is typically confined in a groove to prevent axial displacement by rolling. Fluid pressure along with the contact force between the O-ring and the low-pressure side of the groove exert axial pressure on the O-ring. As a result of this loading, the outside diameter of the O-ring tends to increase and the inside diameter tends to decrease and, because both of these diameters are fixed, the radial contact pressure is increased and the axial length of the seal strips is increased. Thus, in this application the O-ring seal is a ring the outside diameter of which tends to expand and the inside diameter of which tends to contract, with both tendencies restrained by the surrounding concentric cylinders.
For ultra-high vacuum (UHV) systems, those operating in a range below approximately 10.sup.-6 Torr, rubber and elastomers are not suitable as they are permeable to gases to such an extent that a very low base pressure cannot be attained. Further, they contain traces of solvents, plasticizers and other constituents which boil out of them, and such seals cannot survive the high temperatures sometimes necessary to bake out metal ultra-high vacuum systems.
Metallic alloys presently exist which exhibit unique "shape memory" characteristics while undergoing phase transformations between martensitic and austenitic states brought about by passing the alloy through an established temperature range. Such alloys are commonly referred to as "heat recoverable" and include combinations of gold and cadmium, copper and zinc, indium and thallium, copper and tin, and various combinations of nickel and titanium such as those discussed in the referenced Beuhler patent, among others. The nickel-titanium alloys, including those having smaller amounts of other elements, are commonly referred to as "Nitinol," and are commercially available from the Raychem Corporation, Menlo Park, Calif., which also markets Cryoseal.RTM. hermetic seals for semiconductor packages, Cryofit.RTM. mechanical pipe couplings and Cryoplug.RTM. seals for hydraulic components, all of which incorporate Nitinol components. These products, while providing excellent service for their intended purposes, have characteristics inconsistent with UHV system applications. For example, the pipe coupling contains multiple circumferential surfaces in series which plastically deform the coupled pipes at the tips of the surfaces, which can lead to virtual leaks in UHV systems. Such couplings further are not reusable without reworking.
It is desirable to provide a sealing arrangement which advantageously operates in a manner similar to a rubber O-ring and which further is adaptable to the constraints of ultra-high vacuum systems. It is further desirable to provide such an arrangement which is remotely actuatable and reusable.